Radiation cured silicone rubber articles

ABSTRACT

A process for making radiation cured silicone rubber articles is disclosed wherein a hydroxyl-terminated polysilaxane having a molecular weight from about 50,000 to about 2,000,000, optionally modified by mixing with up to 85% of an end-stopped silicone rubber, is mixed with from about 10 to about 70 parts per hundred of rubber of a finely divided silica filler with a particle size in the reinforcing range and other inert fillers as determined by desired final properties; the composition so prepared is formed into the desired shape at room temperature; the article so formed is precured to improve the mechanical properties of the material with which it is made by exposure to ammonia gas, ammonium hydroxide, or to the vapors or solutions of a volatile amine at room temperature; and the precured article is irradiated with high energy electrons or gamma radiation to effect a permanent cure of the material from which the article is formed.

This application is a division of application Ser. No. 211,252, filedNov. 28, 1980, now U.S. Pat. No. 4,362,674.

BACKGROUND

I. Field of Invention

The present invention relates to processes for making radiation curedsilicone rubber articles, and, in particular, to a process whereinarticles formed of silicone rubber are treated substantially immediatelyafter their formation to withstand without serious deformation thestresses involved in the mechanical handling necessary to convey sucharticles through irradiation apparatus.

II. Summary of the Prior Art

Unlike some of the organic rubbers, particularly the newer thermoplasticrubbers, silicone rubber must be crosslinked or vulcanized in order tohave useful properties. The crosslinking process is usually referred to,in the case of silicone rubber as "cure" or "curing." Since theinvention of silicone rubber, the cure has been done by incorporatingfree radical producing catalysts, typically organic peroxides or azocompounds, mixed with rubber and heating the composition to a hightemperature, typically 150°-250° C., for periods ranging from a fewminutes to several hours.

Virtually all commercial, heat-cured silicone rubbers today are curedwith organic peroxides such as benzoyl peroxide, dichlorobenzoylperoxide, tert-butyl perbenzoate or dicumyl peroxide. However, productscured by such curing agents all suffer from a disadvantage in that,after cure has been completed, the products contain chemical residuesfrom the decomposition of the peroxide, and these residues tend toaffect deleteriously properties such as heat-ageing, electricalresistivity and reversion resistance. The presence of these residues,moreover, limits the use of otherwise biologically inert silicone rubberproducts in medical applications, since the residues tend to be leachedout of the rubber by body fluids, saline solutions and other liquidproducts used in medicine.

There are other problems of peroxides curing catalysts. One is that theyare very sensitive to catalyst "poisons" and many common rubbercompounding ingredients, which otherwise would be included in siliconerubber to improve physical strength, flame retardancy, thermal ageing,and so on, cannot be used because they prevent or retard the peroxidecure. For example, most reinforcing carbon blacks, commonly used inorganic rubbers, completely prevent peroxide cure of silicones. Certainperoxide residues, being volatile, cause porosity in the cured rubberand others tend to diffuse out of the compound forming unsightly oilyfilms or crystals on the surface.

More recently, liquid silicone rubbers have been introduced to themarket, which cure by other mechanisms to solid, vulcanized rubberproducts. These liquid rubbers (which occasionally are pastes orsemi-solids) typically cure at room temperature, or at relatively lowtemperatures, and are categorically referred to as "Room TemperatureVulcanizing" (RTV) silicone rubbers. There are several mechanisms forcuring RTV rubbers. Some contain crosslinking agents which are activatedby atmospheric moisture and others are cured by complex catalystscontaining platinum salts. All of them yield chemical byproducts fromthe curing reaction, some of them relatively toxic chemicals such asorganic amines, acetic acid or methanol. They all, therefore, sufferfrom the disadvantages cited for peroxide cure, above.

It has long been known that these disadvantages can be avoided, and thatcured silicone rubber parts free of deleterious catalyst residues can beobtained, by irradiation with high energy electrons or gamma rays, toeffect the cure. For example, the curing of silicone rubber withelectrons was disclosed and claimed in U.S. Pat. No. 2,763,609 to Lewis,et al. This curing process and the benefits resulting therefrom arediscussed in detail in that patent, the disclosure of which isincorporated herein by reference. In spite of the significant advantagesof the irradiation curing process, however, it has never beencommercialized, to the best of our knowledge, because of a seriousobstacle to production of commercial quantities of formed, irradiatedsilicone parts. The problem is that Silicone rubber, like other curablepolymers, must usually be formed into the desired, final shape beforecure because, after cure, it is crosslinked and, therefore, not formableby the usual processes such as molding, extrusion, or casting.

Yet unlike most other polymers, silicone rubber is almost completelylacking in physical strength before cure. Uncured silicone rubber isextremely soft, easily deformed even by working with the bare hands, andit flows readily under low pressures, such as might be experienced whenwinding a silicone tube or sheet on a reel. Cure, whether by peroxidecatalysts or by irradiation, improves the physical strength of siliconerubber dramatically. Tensile strength, for example, is increased 20-50times.

Because of its low precure strength (referred to in the trade as "greenstrength"), it is impossible to convey extruded silicone tubing, orextruded silicone insulated wire, for example, from the extruder to theradiation vault, and to convey it under the electron beam repeatedly,without serious physical damage. If the silicone rubber is to be formedby molding, it is nearly impossible to remove it from the mold withoutdamage, in order to irradiate it. Of course, it cannot beradiation-cured while still in the mold, at least by electronirradiation, because the electrons cannot penetrate the thick metalwalls of a typical mold. This problem has prevented commercialization ofthe radiation cure process.

SUMMARY OF THE PRESENT INVENTION

It is, therefore, an object of this invention to provide a practicalprocess for curing silicone rubber formed objects by the use of highenergy radiation and, particularly, by the use of high energy electrons.

It is a further object to cure silicone rubber without the use of heatenergy, and without the use of chemical agents or catalysts which leavedeleterious residues in the rubber.

A still further object is to effect an apparent cure of silicone rubberformed objects, immediately after the forming process, so that they canbe handled, stored and conveyed through irradiation equipment withoutphysical damage.

The present invention is based on the unexpected discovery that acompound of silicone rubber at least 15% of which has, predominantly,silanol end groups, and a finely divided silica filler with a particlesize in the reinforcing range, is strengthened markedly by a briefexposure, at room temperature, to ammonia gas, or ammonium hydroxide, orto the vapors or solutions of a volatile amine. Although it seems likely(although not proven) that this strengthening does not result frompermanent, chemical crosslinking, but is, rather, an apparent andsomewhat transient cure, the increase in mechanical properties obtainedis more than adequate to enable the formed object, whether it be anextruded tube, wire insulation, thin sheet or molded object, towithstand the severe tensile, compressive and abrasive stresses involvedin the mechanical handling necessary to convey it through irradiationapparatus.

Crosslinking by irradiation with high energy electrons then introducespermanent, primary bond crosslinks. At least in the case of ammonia gasas the precure agent, the ammonia which diffuses into the compound toeffect the precure seems to diffuse back out during, or soon afterirradiation, and is not detectable by sensitive analytical techniques.Thus, crosslinking has been effected without leaving measurable chemicalresidues.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE PRESENT INVENTION

In the preferred embodiment of the invention, a hydroxyl-terminatedpolysiloxane having a molecular weight from about 50,000 to about2,000,000, optionally modified by mixing with up to 85% of anend-stopped silicone rubber polymer, is mixed with from about 10 toabout 70 parts per hundred of rubber of a fumed silica. Other inertfillers may be added to obtain desired property profiles. Suitablemixing equipment includes such high shear mixers as two-roll compoundingmills or sigmablade doughmixers.

The resulting compound is then formed to the desired final shape by anappropriate forming process. That is, tubes or rods are extruded througha die from a rubber extruder; wire insulation is extruded onto bare wirethrough a crosshead extruder; flat sheets are calendered or extrudedthrough a slit die, and so on. Forming is normally carried out at ornear room temperature.

The fabricated rubber is then conveyed immediately through an atmosphereof ammonia or amine vapors, allowing a residence time in the vapors offrom about 10 seconds to about 60 minutes, depending on the samplethickness and degree of cure required. The precuring step is alsopreferably carried out at or about room temperature.

The fabricated rubber, which has now been strengthened greatly by theprecuring process is then crosslinked by exposure to high energyradiation. High energy electrons are preferred as the radiation source,but gamma radiation may also be used. The radiation dose may be fromabout 1 megarad to about 50 megarads with an optimum range from 5megarads to 20 megarads.

For silicone rubber polymers to have useful mechanical properties theymust be compounded with fillers of extremely small particle size,generally less than about 50 mμ in diameter and a high surface area,generally more than 150 M² /g. This type of filler is known as a"reinforcing filler" as opposed to a filler of larger particle size anda smaller surface area which contributes little to the mechanicalstrength of a rubber, but rather serves primarily to reduce the cost ofthe compound. The reinforcing fillers used in silicone rubber aregenerally finely divided silicas, although carbon black fillers may alsobe used. The use of such "reinforcing fillers" is well known in theprior art and is described, for example, at pages 405-423 of"Reinforcement of Elastomers", edited by Gerard Kraus (IntersciencePublishers, New York) in a chapter by John W. Sellers and Frank E.Toonder entitled, "Reinforcing Fine Particle Silicas and Silicates".

For the operation of this invention silica fillers are essential. Fumedsilicas are preferred. Fumed or pyrogenic silica comprises a form ofsilica described in U.S. Pat. No. 2,888,424. Such silicas are sold underthe trade designation Cabosil, by the Cabot Corporation of Boston, Mass.Another type of silica known as silica aerogel, not now soldcommercially, but previously sold under the trade designation Santocelby the Monsanto Chemical Company, is also effective. Silica aerogels aredescribed, for example, in U.S. Pat. Nos. 2,657,149 and 2,093,454.

Although fumed silicas or silica aerogels, or other equally finelydivided silicas are essential to the process disclosed herein, they maybe used in conjunction with other inert fillers or additivesconventionally used to alter specific properties, reduce cost, or aspigments. Such fillers include clays, metal oxides, carbon black, andlarger particle size silicas. Fillers may be process aids (such assilicone oil and a reinforcing filler), flame retardants, pigments,stabilizers, etc. It is well known that fillers may be surface coated orotherwise treated.

In order to illustrate the improved properties attainable with thisinvention the following examples are presented. These examples arepresented by way of disclosure only and should not be construed as beingin any way limiting. In these examples SE-30 refers to a linearend-capped polydimethylsiloxane gum produced by General ElectricCompany; SE-33 refers to a linear end-capped polydimethyl-vinylsiloxanegum produced by General Electric Company; SE-75 refers to a linearnon-end-capped polydimethylsiloxane gum produced by General ElectricCompany; Cabosil MS-7 refers to a pyrogenic, high surface area silicaproduced by the Cabot Corporation; Hisil 233 refers to a medium surfacearea silica produced by PPG Industries, Inc.; Imsil A-10 refers to a lowto medium surface area silica produced by Illinois Minerals Company;Santocel-C refers to a silica aerogel previously produced by theMonsanto Chemical Company; and washed P.D.M.S. refers to apolydimethylsiloxane having, predominantly, silanol end-groups. Thelatter polymers may be made by the KOH-catalyzed polymerization ofoctamethylcyclotetrosiloxane as disclosed in Warrick U.S. Pat. No.2,541,137, issued Feb. 13, 1951 and subsequent removal of potassium ionsby washing the polymer with water, or by other suitable techniques. Suchpolymers will hereinafter be referred to as "hydroxyl-terminatedpolydimethylsiloxanes."

It will, of course, be understood by those skilled in the art that thereactive components are the silanol end-groups, and that the structureof the silicone chain may be varied considerably without departing fromthe scope of this invention. Thus, hydroxyl-terminated polysiloxanes, inwhich some of the methyl groups have been replaced by other aliphatic,aromatic, or cycloaliphatic radicals such as ethyl, vinyl, allyl,phenyl, etc. may also be employed. Methyl-phenyl type copolymers willprobably require relatively higher dosage of irradiation. Similarly someof the methyl groups may be replaced by fluorinated, or otherhalogenated radicals.

The controlled precure which is obtained by exposing hydroxyl-terminatedpolysiloxane rubber compounds to ammonia or amines is highly specific tothe hydroxyl-terminated polymer as is clearly demonstrated by the datapresented hereinbelow in Example I. Most commercial silicone rubber gumsare linear polysiloxanes whose molecules are terminated withtrimethylsilyl or other inert groups. When compounded with a silicafiller, the resulting compounds are completely unaffected by ammonia oramines. On the other hand, a KOH-catalyzed polymer, as disclosed in theWarrick U.S. Pat. No. 2,541,137, made without trimethylsilyl endgroups,and still containing the potassium ions (i.e., not water washed)develops an apparent cure rapidly, when compounded with the silicafiller, to such an extent that it is difficult to form by extrusion,molding, calendering, etc.

However, it is possible to blend conventional silicone rubber polymers,endstopped with trimethylsilyl, or other monofunctional silyl groups,with the hydroxyl-terminated polysiloxane up to 85% of the mixture, andto obtain a useful degree of precure when exposed to ammonia or aminevapors.

EXAMPLE I

In the first series of experiments, various silicone polymers andfillers were mixed together at room temperature on a laboratory two-rollmill. The compounds were then stripped from the mill and pressed into100 mil-thick sheets on a water-cooled laboratory press. The sheets werethen exposed to an atmosphere of anhydrous ammonia for 10 minutes. Theseresults are summarized in Table I.

                                      TABLE I                                     __________________________________________________________________________              A  B  C  D  E  F  G  H  I  J                                        __________________________________________________________________________    SE-33     100   100               50                                          SE-30        100                                                              SE-75              100                                                                              100                                                                              100   100                                                                              50                                          Washed PDMS                 100                                               Cabosil MS-7                                                                             25                                                                               25          25                                                                               30   30                                          Hisil 233        25                                                                               25                                                        Imsil A-10             25                                                     Santocel-C                      30                                            Tensile (psi) Prior                                                                     Nil                                                                              Nil                                                                              Nil                                                                              Nil                                                                              Nil                                                                              Nil                                                                              Nil                                                                              Nil                                                                              Nil                                         to NH.sub.3 Exposure                                                          Tensile (psi) After                                                                     ↓                                                                         ↓                                                                         ↓                                                                         ↓                                                                         ↓                                                                         300                                                                              850                                                                              220                                                                              450                                         NH.sub.3 Exposure                                                             % Elongation After                                                                      ↓                                                                         ↓                                                                         ↓                                                                         ↓                                                                         ↓                                                                         700                                                                              750                                                                              350                                                                              900                                         NH.sub.3 Exposure                                                             __________________________________________________________________________

EXAMPLE II

In this example various amounts of pyrogenic silica were compounded witha non-end blocked polydimethylsiloxane gum (SE-75) and cured withanhydrous ammonia for 10 minutes. These data are summarized in Table II:

                  TABLE II                                                        ______________________________________                                                     A    B       C      D     E                                      ______________________________________                                        SE-75 Non-end Capped                                                                         100    100     100  100   100                                  Polydimethylsiloxane                                                          Polymer (Parts)                                                               Cabosil MS-7 (phr)                                                                            15     25      30   35    45                                  Tensile After 10                                                                              50    300     600  1100  1300                                 Minutes' Exposure to NH.sub.3                                                 (psi)                                                                         % Elongation After 10                                                                        400    700     750  800   750                                  Minutes' Exposure to NH.sub.3                                                 ______________________________________                                    

EXAMPLE III

In this series of experiments, a formulation containing 100 parts of nonend-capped polydimethylsiloxane gum and 40 parts of fumed silica wereexposed to vapors of various organic amines for a period of 10 minutes.Tensile and elongation tests were then performed on these samples. Thedata are summarized in Table III.

                                      TABLE III                                   __________________________________________________________________________                    TENSILE/ELONGATION                                                                          TENSILE/ELONGATION                              COMPOUND  AMINE PRIOR TO EXPOSURE                                                                           AFTER EXPOSURE                                  __________________________________________________________________________    100 g.                                                                            SE-75 Methyl-                                                                             Nil            970-500                                        40 g.                                                                             fumed silica                                                                        Amine                                                               100 g.                                                                            SE-75 Dimethyl-                                                                           Nil           1050/550                                        40 g.                                                                             fumed silica                                                                        Amine                                                               100 g.                                                                            SE-75 Trimethyl-                                                                          Nil           1050/600                                        40 g.                                                                             fumed silica                                                                        Amine                                                               100 g.                                                                            SE-75 Allyl-                                                                              Nil            900/600                                        40 g.                                                                             fumed silica                                                                        Amine                                                               100 g.                                                                            SE-75 Pyridine                                                                            Nil           1050/650                                        40 g.                                                                             fumed silica                                                              __________________________________________________________________________     As can be seen from this data, organic primary, secondary and tertiary        amine vapors may be used to provide a pseudocure to the silicone rubber. 

EXAMPLE IV

In this series of experiments, a formulation composed of 100 partsnon-end capped polydimethylsiloxane gum and 40 parts of fumed silicawere cured with anhydrous ammonia and then irradiated with high-energyelectrons to specified doses. Tensile and elongation tests wereperformed on unaged specimens, specimens aged at 220° C. for 50 hours,and specimens aged in boiling water (100° C.) for 50 hours. These dataare summarized in Table III.

                  TABLE IV                                                        ______________________________________                                        DOSE (Mrads)                                                                                0      2        5       10                                      ______________________________________                                        Initial Tensile                                                                              1200     1100     850   800                                    Strength (psi)                                                                Initial Elongation (%)                                                                       500      420      250   125                                    Tensile After 50 Hours                                                                       450      650      850   750                                    at 220° C. (psi)                                                       Elongation After 50 Hours                                                                    350      325      200    50                                    at 220° C. (%)                                                         Tensile After 50 Hours                                                                       <100     650      850   700                                    in Water at 100° C.                                                    Elongation After 50 Hours                                                                    <50      450      250   100                                    in Water at 100° C.                                                    ______________________________________                                    

As can be seen from these data, a permanent cure is effected byirradiation with high-energy electrons.

EXAMPLE V

In this series of experiments, two formulations were cured withanhydrous ammonia and irradiated by high-energy electrons to the desireddose. Specimens were then subjected to compression at elevatedtemperatures and the % permanent set measured. These results are givenin Table V.

                  TABLE V                                                         ______________________________________                                                                 COMPRESSION                                                        DOSE       SET (%)                                              FORMULATION   (MRADS)    (22 HOURS at 175° C.)                         ______________________________________                                        100 g SE-75                                                                   40 g Cabosil MS-7 2          54                                               NH.sub.3 Cure for 5          38                                               10 Minutes                                                                    100 g SE-75                                                                   50 g Cabosil MS-7 2          33                                               NH.sub.3 Cure for 5          27                                               10 Minutes                                                                    ______________________________________                                    

EXAMPLE VI

In this series of experiments, a compound composed of 100 parts non-endblocked polydimethylsiloxane gum, 40 parts fumed silica (Cabosil MS-7)treated with CF-11 73 (a silicone fluid manufactured by General ElectricCompany) in accordance with U.S. Pat. No.. 2,939,009, 40 partsnon-reinforcing silica (Imsil A-10) and 2 parts red iron oxide wereblended on a two-roll mill and prehardened by exposure to NH₃. Thematerial was then permanently cross-linked by means of high energyelectrons. This material, along with a conventional peroxide curedmaterial (GE 9100-A) was then heat aged at 300° C. for 100, 250 and 350hours. At the end of this time, physical properties were measured inorder to determine the relative heat stability of these materials. Thesedata are summarized in Table VI.

                                      TABLE VI                                    __________________________________________________________________________                      TENSILE/ELONGATION/DUROMETER                                            DOSE         100 HRS.                                                                             250 HRS.                                                                             350 HRS.                               COMPOUND    (MRADS)                                                                             INITIAL                                                                              AT 300° C.                                                                    AT 300° C.                                                                    AT 300° C.                      __________________________________________________________________________    100 parts non-end                                                             blocked PDMS                                                                  40 parts fumed silica                                                         40 parts non-reinforcing                                                      silica                                                                        2 parts Fe.sub.2 O.sub.3                                                                  10     930/160/68                                                                          575/135/67                                                                           475/100/77                                                                           530/65/73                              100 parts non-end                                                             blocked PDMS                                                                  40 parts fumed silica                                                         40 parts non-reinforcing                                                      silica                                                                        2 parts Fe.sub.2 O.sub.3                                                                  20     950/100/72                                                                          470/75/70                                                                            465/50/85                                                                            515/50/80                              Peroxide cured                                                                            (Peroxide                                                                           1000/400/55                                                                          545/100/75                                                                            460/0/97                                                                             700/0/96                              polymethylvinylsiloxane                                                                   cure)                                                             (GE 9100-A)                                                                   Peroxide cured                                                                            (Peroxide                                                                           1100/400/59                                                                          690/75/85                                                                            600/30/93                                                                             550/0/95                              polymethylvinylsiloxane                                                                   cured)                                                            (GE SE-9058)                                                                  __________________________________________________________________________

As can be seen from these data, silicone crosslinked by high energyelectrons appears to have superior heat stability (based on %elongation) as compared to peroxide crosslinked silicone.

It will be understood that the embodiments, product, process, andpractices described and portrayed herein have been presented by way ofdisclosure, rather than limitation, and that various modifications,substitutions, and combinations may be effected without departure fromthe spirit and scope of this invention in its broader aspects.

We therefore claim:
 1. An irradiated silicone rubber article produced inaccordance with the following process:(a) forming into a desired shape amixture of (i) silicone gum which includes a substantial amount of oneor more hydroxyl-terminated polydimethylsiloxanes having a molecularweight from about 50,000 to about 2,000,000 and in which some of themethyl groups may have been replaced by other organic aliphatic,aromatic, or cycloaliphatic radicals such as ethyl, vinyl, allyl,phenyl, or halogenated radicals of the foregoing types, and (ii) about10 to about 70 parts per hundred of rubber of a finely divided silicafiller with a particle size in the reinforcing range: (b) treating themixture so formed with a precuring agent selected from the groupconsisting of ammonia gas, ammonium hydroxide, vapors of a volatileamine, or a solution of a volatile amine; and (c) irradiating with highenergy ionizing radiation the precured shape.